1. Field of the Invention
The present invention relates to mobile communications. More particularly, the present invention relates to a method and an apparatus for soft buffer processing of downlink data transmitted based on a Hybrid Automatic Repeat Request (HARQ).
2. Description of the Related Art
A Long Term Evolution (LTE) system supports a working mode of Time Division Duplex (TDD).
FIG. 1 illustrates a frame structure of a TDD system according to the related art.
Referring to FIG. 1, each wireless frame has a length of 10 ms, which is bisected into two half-frames with a length of 5 ms. Each half-frame includes eight time slots with a length of 0.5 ms and three special fields, i.e., a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP), and an Uplink Pilot Time Slot (UpPTS), the sum of the length of which is 1 ms. Each subframe is formed of two continuous time slots, i.e., the kth subframe includes time slot 2k and time slot 2k+1. The TDD system supports seven different uplink and downlink configurations as shown in FIG. 1. Here, D represents a downlink subframe, U represents an uplink subframe, and S represents the special subframe including three special fields. Table 1 illustrates Uplink and Downlink Configurations of LTE TDD.
TABLE 1Conver-sionConfigura-PointSubframe No.tion No.Period01234567890 5 msDSUUUDSUUU1 5 msDSUUDDSUUD2 5 msDSUDDDSUDD310 msDSUUUDDDDD410 msDSUUDDDDDD510 msDSUDDDDDDD610 msDSUUUDSUUD
To improve users' transmission rate, in an enhanced LTE-Advanced (LTE-A) system of the LTE system, a greater working bandwidth is obtained from aggregating a plurality of Component Carrier (CC), i.e., Carrier Aggregation (CA), and uplink and downlink of a communications system is constituted, thereby supporting a higher transmission rate. For example, support for a bandwidth of 100 MHz is obtained by aggregating five CCs of 20 MHz, wherein each CC is referred to as a Cell. For a User Equipment (UE), a base station can configure the UE to work in a plurality of downlink Cells, wherein one Cell is a Primary cell (Pcell) while other Cells are referred to as Secondary cells (Scells).
In Release 10 of the LTE TDD system, a plurality of Cells which are restricted to be aggregated together use the same uplink and downlink configuration, such that when a HARQ transmission timing is processed, it is possible to completely reuse the timing relation of a HARQ defining one Cell in LTE, thereby requiring no additional standardization work. Specifically, similar to Release 8 of LTE the TDD, a Physical Downlink Control Channel (PDCCH) is used for scheduling a Physical Downlink Shared Channel (PDSCH) within the current subframe for HARQ transmission of downlink data. One uplink subframe n can feedback the ACKnowledgement/Non-ACKnowledgement (ACK/NACK) information corresponding to the PDSCH or the PDCCH of downlink Semi-Persistent Scheduling (SPS) release in zero, one or a plurality of downlink subframes, the index of these downlink subframes being n-k, wherein k belongs to set K which is determined by uplink and downlink configurations and uplink subframe n, as shown in Table 2.
TABLE 2Index Set KUplink andDownlinkConfigura-Subframe Index ntions01234567890——6—4——6—41——7, 64———7, 64—2——8, 7,————8, 7,——4, 64, 63——7, 6,6, 55, 4—————114——12, 8,6, 5,——————7, 114, 75——13, 12,———————9, 8,7, 5,4, 11, 66——775——77—
According to the timing relation of HARQ, in Release 8/9/10 of LTE TDD, the maximum numbers of downlink HARQ processes corresponding to the above seven TDD uplink and downlink configurations are different. Here, the maximum number of downlink HARQ processes to each TDD uplink and downlink configuration ensures that the base station can be indexed by HARQ processes of the PDCCH and can identify respective parallel HARQ processes without confusion.
TABLE 3Maximum Number of downlink HARQ processesUplink andMaximum Number ofdownlinkdownlink HARQconfigurationsprocesses0417210394 125 156 6
Table 3 illustrates the timing relation of HARQ in Release 10 of LTE TDD. Another problem relating to HARQ is how to process a soft buffer. A UE is divided into a plurality of classes according to its processing capacity, based on whether or not the UE supports Multiple-Input Multiple-Output (MIMO), the number of the maximum data stream of the supported MIMO, the size of soft buffer, and the like. Here, the soft buffer is used to store the received soft bits, when the UE fails to correctly decode the data sent by the base station, and enables soft combining during HARQ retransmission, thereby improving link performance. The processing of the soft buffer affects Rate Matching (RM) for downlink data. In Release 10 of LTE TDD, the soft buffer of the UE is described as Nsoft, and no matter whether the UE is in a single carrier mode or a CA mode, RM is performed for each code block of one transport block according to the size of soft buffer
            N      cb        =          min      ⁡              (                              ⌊                                          N                IR                            C                        ⌋                    ,                      K            w                          )              ,wherein C is the sum of the code blocks divided by the transport block,
            N      IR        =          ⌊                        N          soft                                      K            C                    ·                      K            MIMO                    ·                      min            ⁡                          (                                                M                                      DL                    ⁢                    _                    ⁢                    HARQ                                                  ,                                  M                  limit                                            )                                          ⌋        ,KMIMO depends on the transmission mode of the UE, KMIMO=2 for the MIMO transmission mode, KMIMO=1 for the non-MIMO transmission mode, MDL_HARQ is the maximum number of downlink HARQ processes set forth in the above Table 3, Mlimit is a constant 8, Kc is a constant relating to UE category, and K is the sum of the encoding bits transmitted by turbo codes. Specifically, no matter which carriers the UE works in, RM is performed according to the condition that the UE only configures the current one carrier. When the UE configures a plurality of Cells, the processing result is that the hypothetical HARQ soft buffer for one code block in RM may be greater than the soft buffer capacity that the UE can support. In Release 10 of LTE TDD, suppose that the UE equally divides its soft buffer to a plurality of Cells. In order to better support HARQ Incremental Redundancy (IR), the base station should know which soft bits the UE stores when the UE fails to correctly decode one code block. Therefore, the number of the carrier configured by the UE is described as NcellsDL, and for each Cell and at least KMIMO·min (MDL_HARQ, Mlimit) transport blocks, when one code block of one transport block fails to decode, it is regulated in LTE-A that the UE at least needs to store soft bits wk wk+1, . . . , wmod(k+nsB−1, Ncb) for this code block, wherein
            n      SB        =          min      (                        N          cb                ,                  ⌊                                    N              soft              ′                                      C              ·                              N                cells                DL                            ·                              K                MIMO                            ·                              min                ⁡                                  (                                                            M                                              DL                        ⁢                        _                        ⁢                        HARQ                                                              ,                                          M                      limit                                                        )                                                              ⌋                    )        ,wk is a soft bit received by the UE, and k is a smaller index in the indexes of respective soft bits received by the UE.
As the distance of the frequency domain between a plurality of Cells that perform carrier aggregation is large enough, these Cells entirely can use different uplink and downlink configurations without interference to each other. Thus, in the subsequent study of LTE-A, a very significant research project is how to efficiently support different uplink and downlink configurations of a plurality of Cells. Since the uplink and downlink configuration for UE is different in different Cells, there will be a structure where some Cells are uplink subframes while others are downlink subframes in the same subframe. This is different from Release 10 of LTE TDD and accordingly, the timing relation of HARQ may need to be re-defined.
Therefore, a need exists for a method and an apparatus for soft buffer processing of data in HARQ transmission.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.